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International external quality assessment scheme for the laboratory diagnosis of diphtheria

KATHRYN H. ENGLER, ROMAN S. KOZLOV^*, STEVEN J. COPPING, MEMBERS OF THE EUROPEAN LABORATORY WORKING GROUP ON DIPHTHERIA and ANDROULLA EFSTRATIOU

Respiratory and Systemic Infection Laboratory and Quality Assurance Laboratory, PHLS Central Public Health Laboratory, 61 Colindale Avenue, London NW9 5HT and *Institute of Antimicrobial Chemotherapy, Smolensk State Medical Academy, Smolensk, Russia

Corresponding author: Dr K. H. Engler (e-mail: kengler{at}phls.nhs.uk).

Received 23 March 2001; accepted 14 May 2001.

Abstract

An international external quality assessment (EQA) scheme has been established so as to evaluate the proficiency of specialist, national diphtheria reference laboratories in the laboratory diagnosis of diphtheria. Six simulated clinical specimens were freeze-dried and distributed to 23 participants in 20 countries. Participants were asked to isolate, identify and perform toxigenicity testing on any corynebacteria present and to complete a simple questionnaire describing the procedures and reagents used. Only three laboratories obtained correct biochemical and toxigenicity results for all six specimens. The majority of laboratories performed better with toxigenicity testing than with the biochemical identification. Of concern were the results from three laboratories that failed to isolate any corynebacteria from four or more of the specimens. In one centre this was shown to be due to lack of availability of ‘in-date’ media and, in the other two, was presumed to be due to lack of experience in primary laboratory diagnostics for this organism. It is essential that countries, globally, maintain awareness and laboratory capabilities in this specialised area of microbiology. EQA is an invaluable process, which enables laboratories to monitor, evaluate and improve their own performance in such areas.